CMYK
The four color of inks used in traditional printing (offset) to achieve any other color (various screen percentages of Cyan, Magenta, Yellow and Black (yes, I know that does not start with a “K”)).

Pantone Ink (PMS)
Pantone is a company that creates an exceptionally large palette of inks that can be matched precisely. One PMS (Pantone Matching System) ink color may achieve a much cleaner and more precise color than mixing various screens of CMYK.

RGB
On computer and television monitors, color is achieved by mixing light…in very much the same way the human eye views light. Red, Green and Blue are the “cones” that the human eye uses to perceive the color of reflected light. Various percentages of RGB create the colors you see online.

“Browser-safe” Web Colors
Refers to the more limited RGB palette that was viewable by older 256-color monitors. The browser-safe color wheel contains very limited shades of colors that were viewable by the typical web browser (the application, not a person). The Browser-Safe Palette only contains 216 colors out of a possible 256. That is because the other 40 colors vary on Macs and PCs. (bummer)

HEX Web Color Code
Now that so many people have monitors that can view millions of colors, a web site designer may more accurately create a broader range of colors. Almost any color can be created using what is called Hex (hexadecimal) color code. Using a 6-digit Hex code (comprised of letters and numbers for each of red, blue and green values).

AND, sadly...

Even on the same operating system, variations in monitor quality and calibration makes precise color accuracy/consistency impossible on all monitors.

Although it is confusing (for me anyway, and I use and test colors online and cross-platform everyday), I tend to stick with HEX color codes and stay away entirely from old HTML color names....nor do I use the “browser-safe” color wheel/palette.

Search the term “Web Color” on Wikipedia for lots of color palette resources and color codes.